222 SCIENCE IN SHORT CHAPTERS. 



When such a spat in the course of the sun's rotation reaches 

 that part which forms the visible edge of the sun, it must, 

 if rendered visible, be seen as a notch; but what will be 

 the depth of such a notch? Only about 1-4.3'Oth of the 

 sun's diameter. But the apparent depth would be much 

 less as the edge or rim of the spot next to the observer 

 would cut off more or less of its actually visible depth, this 

 amount depending upon the lateral or east and west dia- 

 meter of the spot and its position at the time of observa- 

 tion. 



Thus, the visible depth of such a notch would rarely ex- 

 ceed one thousandth of the sun's apparent diameter, or 

 might be much less. The sun being globular, the edge 

 which is visible to us is but our horizon of his fiery ocean, 

 which we see athwart the intervening surface as it gradually 

 bends away from our view. So small an indent upon this 

 edge would, under ordinary circumstances of observation, 

 be rendered quite invisible by the irradiation of the vast 

 globular surface of the glaring photosphere, upon which it 

 would visually encroach. 



If, however, this body of glare could be screened off, and 

 only a line of the sun's edge, less than one thousandth of 

 his diameter, remain visible, the notch would appear as a 

 distinct break in this curved line of light. If a group of 

 spots, or a great irregular spot with several umbrae, were at 

 such a time situated upon the sun's edge, the appearance of 

 a series of such notches or breaks leaving intermediate de- 

 tachments of the visible ring of the photosphere would be 

 the necessary result, and thus would be presented exactly 

 the appearance described as "Baily's beads." 



I have been led to anticipate a display of these beads 

 during the late eclipse by the fact that some days preceding 

 it a fine group or spots visible to the naked eye through 

 a London fog were traveling towards the eastern edge of 

 the sun, and should have reached the limb at about the 

 time of the eclipse. The beads were observed by the Rev. 

 S. J. Perry just where I expected them to appear. I have 

 not yet learnt on which side of the sun they were observed 

 and photographed by Lord Lindsay. 



Baily's first observation of the beads was made during 



